Chromium dioxide is a material of high magnetic characteristics which finds its prevailing application in the field of magnetic tape recording. It shows a certain chemical reactivity towards water and certain organic substances, particularly towards those containing easily oxidizable functional groups such as for instance hydroxyl or aminic groups. Said functional groups are often present in the resins used in the formulations for magnetic recording tapes, wherefore the CrO.sub.2 particles incorporated in the tape may oxidate said functional groups, reducing themselves, at least superficially, to chromium compounds of a lower valency, such as CrOOH, which are not ferromagnetic and in consequence whereof the residual magnetization of the magnetic tape may drop with time, resulting in a reduction of the output level of the tape on which a recording has been made.
It is known to stabilize the CrO.sub.2 subjecting the surface of its particles to a reducing treatment, by making it react, for instance, with H.sub.2 S or with alkaline bisulphites. Said treatment has however the drawback of consuming, through the reduction reaction, a considerable quantity of treated CrO.sub.2 (up to about 30%).
It has also been suggested to stabilize the CrO.sub.2 by coating its particles with various water-insoluble substances, as for instance SiO.sub.2 or Al.sub.2 O.sub.3. However, no appreciable stabilizing effect could be obtained with such coatings: while the CrO.sub.2 particles, dispersed in water after the treatment, showed a considerable reduction of their reactivity with water, no appreciable improvement could be detected when the stability of the CrO.sub.2 was evaluated in the tapes themselves.